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  <meta property="og:description" content="React v0.4 is very close to completion. As we finish it off, we&#39;d like to share with you some of the major changes we&#39;ve made since v0.3. This is the first of several posts we&#39;ll be making over the next week.
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<h1>

  New in React v0.4: Autobind by Default

</h1>

<p class="meta">
  July  2, 2013
  by
  
    
      <a href="https://twitter.com/zpao">Paul O’Shannessy</a>
    
    
  
</p>

<hr>

<div class="post">
  <p>React v0.4 is very close to completion. As we finish it off, we&#39;d like to share with you some of the major changes we&#39;ve made since v0.3. This is the first of several posts we&#39;ll be making over the next week.</p>

<h2>What is React.autoBind?</h2>

<p>If you take a look at most of our current examples, you&#39;ll see us using <code>React.autoBind</code> for event handlers. This is used in place of <code>Function.prototype.bind</code>. Remember that in JS, <a href="https://bonsaiden.github.io/JavaScript-Garden/#function.this">function calls are late-bound</a>. That means that if you simply pass a function around, the <code>this</code> used inside won&#39;t necessarily be the <code>this</code> you expect. <code>Function.prototype.bind</code> creates a new, properly bound, function so that when called, <code>this</code> is exactly what you expect it to be.</p>

<p>Before we launched React, we would write this:</p>
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
  <span class="nx">onClick</span><span class="o">:</span> <span class="kd">function</span><span class="p">(</span><span class="nx">event</span><span class="p">)</span> <span class="p">{</span><span class="cm">/* do something with this */</span><span class="p">},</span>
  <span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="hll">    <span class="k">return</span> <span class="o">&lt;</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">onClick</span><span class="p">.</span><span class="nx">bind</span><span class="p">(</span><span class="k">this</span><span class="p">)}</span> <span class="o">/&gt;</span><span class="p">;</span>
</span>  <span class="p">}</span>
<span class="p">});</span>
</code></pre></div>
<p>We wrote <code>React.autoBind</code> as a way to cache the function creation and save on memory usage. Since <code>render</code> can get called multiple times, if you used <code>this.onClick.bind(this)</code> you would actually create a new function on each pass. With React v0.3 you were able to write this instead:</p>
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
<span class="hll">  <span class="nx">onClick</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">autoBind</span><span class="p">(</span><span class="kd">function</span><span class="p">(</span><span class="nx">event</span><span class="p">)</span> <span class="p">{</span><span class="cm">/* do something with this */</span><span class="p">}),</span>
</span>  <span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="hll">    <span class="k">return</span> <span class="o">&lt;</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">onClick</span><span class="p">}</span> <span class="o">/&gt;</span><span class="p">;</span>
</span>  <span class="p">}</span>
<span class="p">});</span>
</code></pre></div>
<h2>What&#39;s Changing in v0.4?</h2>

<p>After using <code>React.autoBind</code> for a few weeks, we realized that there were very few times that we didn&#39;t want that behavior. So we made it the default! Now all methods defined within <code>React.createClass</code> will already be bound to the correct instance.</p>

<p>Starting with v0.4 you can just write this:</p>
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
<span class="hll">  <span class="nx">onClick</span><span class="o">:</span> <span class="kd">function</span><span class="p">(</span><span class="nx">event</span><span class="p">)</span> <span class="p">{</span><span class="cm">/* do something with this */</span><span class="p">},</span>
</span>  <span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="hll">    <span class="k">return</span> <span class="o">&lt;</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">onClick</span><span class="p">}</span> <span class="o">/&gt;</span><span class="p">;</span>
</span>  <span class="p">}</span>
<span class="p">});</span>
</code></pre></div>
<p>For v0.4 we will simply be making <code>React.autoBind</code> a no-op — it will just return the function you pass to it. Most likely you won&#39;t have to change your code to account for this change, though we encourage you to update. We&#39;ll publish a migration guide documenting this and other changes that come along with React v0.4.</p>

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